1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In general, regardless of differences in a developing system, an image forming apparatus by a conventional electrophotographic method forms an image by: charging uniformly a drum-shaped or a belt-shaped image bearing member while rotating it; forming a latent-image pattern on the image bearing member with a laser light; forming a visible image from it by a developing apparatus; and further transferring a toner image on a transfer medium.
In general, after a transfer step, a toner component and so on remaining on the image bearing member is removed in a cleaning step to have a surface of the image bearing member in a sufficiently clean state, and then charging is carried out.
However, in recent years, colorization of an output image is in progress, and a toner having a smaller diameter and a larger sphericity is desired for high-quality image and stable image quality. When such a toner is used in an electrophotographic image forming method, it is difficult to remove the toner in the cleaning step, and challenges for cleaning are getting larger. After a toner image is transferred on the transfer medium and if a toner component not transferred remains on the image bearing member, the residual of the toner component is conveyed directly to the charging step. This is a problem that it inhibits uniform charging of the image bearing member.
There is a method to increase rubbing power of a cleaning member against the image bearing member in order to remove the toner having a smaller diameter and a larger sphericity in the cleaning step. In this case, however, there is a problem that abrasion of the image bearing member or the cleaning member significantly advances.
Also, factors of the adhesion of the toner to the image bearing member also include the charging step.
In recent years, as a charging method of an electrophotographic image forming apparatus, an AC superimposed charging system that an alternating-current (AC) component is superimposed on a direct-current (DC) component has been used. A close charging system by the AC superimposed charging system can downsize the apparatus while achieving high-quality image. At the same time, since it may have the charging member and the image bearing member in a non-contact state while maintaining uniformity of the charge, there is no minor unevenness at a contact between the charging member and a surface of the image bearing member or gap variation between the charging member and the surface of the image bearing member, and as a result, degradation of the charging member may be suppressed.
However, when the image bearing member is an organic photoconductor (OPC), energy of the AC superimposed charging cuts resin chains at the surface of the image bearing member, which reduces mechanical strength, and there is a problem that the image bearing member wears significantly. Since the superimposed charging activates the surface of the image bearing member, adhesive force between the surface of the image bearing member and a toner increases, and there is also a problem that cleanability against the image bearing member decreases.
Accordingly, there exist electrical stresses and physical stresses in the steps of image formation by the electrophotographic method, and the image bearing member which has received these stresses have a surface condition thereof changed over time due to use.
For such a problem, technologies to apply a protective agent on a surface of the image bearing member have been proposed.
For example, a method that a block-shaped protective agent having zinc stearate as a main component, a so-called protective agent block, is applied on a surface of an image bearing member (see Japanese Patent Application Publication (JP-B) No. 51-22380), a method that a protective agent block having zinc stearate as a main component with an addition of boron nitride is applied on a surface of an image bearing member (see Japanese Patent Application Laid-Open (JP-A) No. 2006-350240) and so on are proposed.
Application of the protective agent block on the surface of the image bearing member reduces a frictional coefficient of the surface of the image bearing member and reduces degradation of a cleaning blade or the image bearing member, and at the same time, it improves detachment of deposits adhering to the surface of the image bearing member such as toner components not transferred. As a result, it is possible to prevent cleaning defects and occurrence of filming over time.
Also, as a technique to apply the protective agent block on the surface of the image bearing member, a protective layer forming apparatus including: a protective agent block; a protective agent supplying member including a brush-shaped rotary member, which contacts the protective agent block to adhere it on a surface thereof and applies a protective agent on an image bearing member; and a protective agent pressurizing member, which pressurizes the protective agent block so that it contacts the protective agent supplying member is proposed (see JP-A No. 2007-65100 and JP-A No. 2007-293240).
However, there is a problem with these proposed technologies that, depending on a rotation of the brush-shaped rotary member, powder of the protective agent rubbed from the protective agent block flies in a large quantity, resulting in a large quantity of the protective agent being wasted. Also, brush fibers get flattened or degraded over time, and there is a problem that a predetermined amount of the protective agent cannot be supplied over a long period of time because of unstable consumption of the protective agent.
Therefore, a technology to use a roller-shaped protective agent supplying member having a foam layer as a protective agent supplying member of a protective layer forming apparatus is proposed (see JP-A No. 2009-150986). With this proposed technology, powder of a protective agent by rubbing hardly flies.
However, with this proposed technology, since the foam layer is composed of isolated cells, the foam layer is degraded or destructed over time due to rubbing with the protective agent block or the image bearing member. As a result, there is a problem of filming of the image bearing member due to insufficient supply of the protective agent to the image bearing member over a long period of time.
Accordingly, there currently is being asked to provide an image forming apparatus having a relatively simple configuration and capable of preventing filming of the toner consumption even with a small consumption of a protection agent even when a protective agent supplying member having a foam layer is used.